In recent years, various compact fluorescent lamps have been introduced or otherwise proposed as an alternative to incandescent lamps for general illumination. One example of a compact fluorescent lamp is one which is generally referred to as a negative glow discharge lamp. Such a lamp is typically comprised of a bulbous envelope containing a noble gas and mercury with a phosphor coating on an inner surface of the envelope. The lamp includes a pair of electrodes typically spaced about 1 to 3 centimeters apart. Examples of typical negative glow discharge lamps are found in U.S. Pat. No. 4,904,900 to Bouchard et al and U.S. Pat. No. 5,027,030 to Bouchard et al.
It is well-known that the light output of a low-pressure mercury vapor discharge lamp is a function of the mercury vapor pressure, which in turn often depends upon the temperature of the coldest region of the glass envelope of the lamp. It is further known that the envelope cold spot temperature for most efficient lamp operation is approximately 40.degree. C., which causes a mercury vapor pressure of approximately 4 to 6.times.10.sup.-3 torr to occur inside the lamp. In many installations, particularly in almost completely enclosed fixtures or where the ambient temperature rises very high at times, the lamp may be required to operate at temperatures considerably above 40.degree. C. Such situations cause an increase in the mercury vapor pressure and a consequent reduction in light output.
To control the mercury vapor pressure within a prescribed range during such extreme situations, it is known to enclose an amalgam or an amalgam-forming material at various locations within the lamp envelope. Typical examples of compact fluorescent lamps containing an amalgam are shown and described in UK Patent Application GB2157883A and European Patent Application 0327346A2. In both of these references, a quantity of amalgam is present in an exhaust tube behind the electrode. In the latter reference a quantity of an auxiliary amalgam is also fixed to one of the lead wires.
While lamps employing an amalgam or an amalgam-forming material for regulating the internal mercury vapor pressure operate at elevated temperatures more efficiently than ordinary lamps, they suffer from the inherent drawback of lower efficiency operation at normal or low ambient temperatures. At low ambient temperatures, the vapor pressure of the mercury above the amalgam is too low to cause an efficient generation of light. Moreover, the low mercury vapor pressure associated with cold weather operation can cause destructive sputtering of the lamp cathode because of associated high cathode fall voltages.
U.S Pat. No. 3,336,502, which issued to Leland W. Gilliatt on Aug. 15, 1967, describes a mercury vapor discharge lamp containing a mercury-amalgamative metal, such as a ring of indium, on the inner surface of the lamp envelope. A heater 6 in the form of a collar embraces the lamp outside the indium ring. The heater is designed to heat the portion of the lamp at the amalgam ring to its optimum operating temperature near or at 140.degree. F. although the ambient temperature is considerably lower. With particular attention to the embodiment depicted in FIG. 6 of the Gilliatt patent, a thermostatic switch 24 is connected between a power terminal 16 and a heater terminal 17 and located adjacent the lamp 1. The switch is adjusted to close as the ambient temperature adjacent the lamp drops below about 130.degree. F.
While the above-described heater collar and switch may be effective, the presence of a heater collar disposed adjacent the external surface of the lamp reduces the light output from the lamp and creates a dark ring at the center of the lamp. Moreover, replacement of a failed lamp is complicated by the need to remove the heater collar from a failed lamp and then place it around a new lamp.
U.S. Pat. No. 3,859,555, which issued to Latassa et al on Jan. 7, 1975, describes a fluorescent lamp wherein an amalgam-forming material is disposed on the surface of a positive-temperature-coefficient thermistor electrically connected across one of the lamp electrodes. While this approach is effective at regulating the mercury vapor pressure of the lamp substantially independent of the ambient temperature of the lamp, a small amount of current is required to keep the thermistor heated resulting in a unnecessary use of power. In addition to consuming power during lamp operation, the thermistor is relatively expensive.